1. Field of the Invention
The present invention relates to a device and method for detecting a force factor of a loudspeaker and, more particularly, to a device and method that are utilized by end users so as to accurately detect the force factor of a loudspeaker.
2. Description of Related Art
To protect the physical structure of a loudspeaker from being permanently damaged, it is a practice not to directly drive a loudspeaker with a linearly-amplified audio signal, which may, if the driving signal is too large, cause greater diaphragm excursion or even exceed beyond the displacement limit of the diaphragm excursion, thus leading to a change in the property, or a shorter lifetime, of the diaphragm of the loudspeaker, or even direct damage to the structure of the diaphragm. On the other hand, to have better listening experience, one may put the output volume of the loudspeaker to its limit, which may stress the diaphragm excursion of the loudspeaker to the displacement limit. Therefore, it has become an issue of the design of a loudspeaker and its driving circuit on how to detect, or predict, the diaphragm excursion so as to make an optimal tradeoff between the output volume and the protection of the loudspeaker.
FIG. 1 shows an equivalent circuit diagram of a prior-art loudspeaker 100 having two terminal inputs 110. By applying a driving voltage u at the two terminal inputs 110, the diaphragm of the loudspeaker 100 is induced to vibrate so as to generate human-perceivable sound waves. In the equivalent circuit of the loudspeaker 100, the circuit of the electrical impedance and the back electromotive force (BEMF) parallels the aspect of the electrical property of the loudspeaker 100, while the circuit of the electromagnetic force, mechanical impedance and saturation electromagnetic force parallels the aspect of the mechanical property of the loudspeaker 100.
The driving voltage u at the terminal inputs 110 forms a current i. In the aspect of the mechanical property of the loudspeaker 100, an electromagnetic force with a magnitude of Φ*i is formed due to the induction caused by the current i, where Φ is the force factor, which is a characteristic parameter of the loudspeaker, and the electromagnetic force causes a velocity of displacement v on the diaphragm of the loudspeaker with a mechanical impedance Zm. The saturation electromagnetic force is the part of the induced electromagnetic force when the diaphragm excursion of the loudspeaker 100 is close to or greater than the displacement limit. The magnitude of the saturation electromagnetic force is M*v′, where v′ is the first derivative of the velocity of displacement v, and the coefficient M is approaching zero when the diaphragm excursion is at a low value. The parameters described hereto can be related by the equation as follows:Φ·i=Zm·v+M·V  (1)
The function of the velocity of displacement v can be derived from Eq. (1). As shown in FIG. 1, the equivalent circuit has a BEMF with magnitude of Φ*v and indicates that the driving voltage u is not fully applied on the electrical impedance Ze; instead, the mechanical aspect of the loudspeaker generates a voltage of the BEMF with magnitude of t*v, where the voltage is connected to the electrical impedance Ze in series. Therefore, with the known driving voltage u, one can obtain the magnitude of the BEMF Φ*v by measuring the current i. However, the diaphragm excursion (i.e., the integral of the velocity of displacement v) cannot be obtained without first computing the magnitude of the force factor Φ.
During the warm-up calibration of the loudspeaker 100, one can detect the magnitude of the force factor Φ by performing a reverse computation on the diaphragm excursion x of the loudspeaker 100 when operating at the displacement limit. In the prior art, there exist two approaches to check how close the diaphragm excursion is to the displacement limit. One approach is to analyze the electrical signal, for example, the driving voltage u or the total harmonic distortion (THD) of the current i. When the diaphragm excursion x of the loudspeaker 100 is close to or greater than the displacement limit, however, the THD measured by an electrical signal may not be distinguishable because most of the non-linearity of the loudspeaker 100 occur in the resonant frequency, but not in the harmonic frequency of the electrical signal. The other approach is to analyze the THD on the sound pressure level (SPL) generated by the loudspeaker. The THD on SPL is more distinguishable, but the measurement of the SPL is only feasible under a controlled environment, and therefore the measurement is conducted in the lab or a factory. Besides, the measurement of the SPL requires some special instruments, which may not be easily accessible to the end users.